A volcanic crater several times larger than one found at
Hawaii's Kilauea volcano has been photographed on Jupiter's moon
Io during a close flyby performed by NASA's Galileo spacecraft.

"It appears that the Prometheus volcano on Io has
characteristics remarkably similar to those of the Kilauea
volcano in Hawaii, although Prometheus is much larger," said Dr.
Laszlo Keszthelyi (KEST-ay), a Galileo research associate at the
University of Arizona, Tucson, AZ. "Both volcanoes are long-
lived eruptions, with flows that apparently travel through lava
tubes and produce plumes when they interact with cooler
materials."

The sharp images of Prometheus released today come from two
of Galileo's onboard instruments -- the camera, and the near-
infrared mapping spectrometer which observes in wavelengths not
visible to the naked eye. The images were taken during the close
flyby of Io by Galileo on October 10, 1999, and are part of a
large batch of data currently being transmitted to Earth.

"We've been having a feast looking at the material from Io,"
said Dr. Rosaly Lopes-Gautier NASA's Jet Propulsion Laboratory,
Pasadena, CA. "We have been waiting for such high-resolution
images of Io for more than 10 years." Scientists will present an
assortment of new images and describe their latest discoveries at
a press briefing scheduled for November 19 at NASA Headquarters,
Washington, DC.

Prometheus is the "Old Faithful" of Io's many volcanoes. It
has been active during every observation over the past 20 years
by NASA's Voyager and Galileo spacecraft and the Hubble Space
Telescope. The new spectrometer images show two distinct hot
spots at Prometheus -- a large one to the west and a fainter,
cooler one to the east. The images reveal numerous lava flows
near the western hot spot and enable scientists to identify a
crater, or caldera, 28 kilometers (17 miles) long and 14
kilometers (9 miles) wide near the hot spot to the east.

Previously, it was thought that the 50 to 100 kilometer- (30
to 60 mile-) tall plume observed at Prometheus formed where the
lava erupts onto the surface. Now, however, it now appears that
the plume forms at the far end of the lava flows. The caldera
and eastern hot spot are thought to be associated with the vent
where the molten rock rises to the surface. It appears that
after the lava reaches the surface, it is transported westward
through lava tubes for about 100 kilometers (60 miles) before
breaking out onto the surface again. Here, numerous lava flows
wander across a plain covered with sulfur dioxide-rich snow. The
plume is created by the interaction of the hot lava with the
snow.

This plume feature is just one of several similarities
between Prometheus and Hawaii's Kilauea. Volcanologists say that
Prometheus has been erupting for more than 20 years and Kilauea
has been erupting for more than 16 years. The current vent at
Kilauea consists of a small lava lake about 100 meters (330 feet)
across that produces a relatively small thermal hot spot. From
this vent, lava is transported 10 kilometers (6 miles) in lava
tubes to the Pacific Ocean where large steam plumes are generated
by the interaction between the hot lava and the ocean. Galileo
scientists believe the plume seen on the western end of
Prometheus is similar to this Hawaiian steam plume, except the
Ionian plume is composed largely of sulfur dioxide and rises much
higher because of Io's low atmospheric density and gravity.

Another Io flyby, this time at an altitude of 300 kilometers
(186 miles), is planned for November 25 at 8:40 p.m. Pacific Time
(11:40 p.m. Eastern Time). (Times given are in Earth-received
time -- or the time when the signal of the event is received on
Earth.) The Io flybys are challenging and risky, because Io lies
in an area of intense radiation from Jupiter's radiation belts,
and that radiation can harm spacecraft components. Because of
the risk, the flybys were scheduled for the final portion of
Galileo's extended mission.

The spacecraft has been orbiting Jupiter and its moons for
nearly four years, with its primary mission running from December
1995 until December 1997, followed by its current two-year
extended mission. JPL manages the Galileo mission for NASA's
Office of Space Science, Washington, D.C. JPL is operated for
NASA by the California Institute of Technology, Pasadena, CA.